In the related art, an electric power steering device configured to detect a steering torque applied to a steering wheel by a driver or the like using a steering torque sensor, and generate a steering assist force according to the detected steering torque using a motor to apply the steering assist force to the steering wheel is known. In recent times, an electric power steering device capable of obtaining a larger steering assist force by boosting a voltage of a battery using a booster circuit and supplying the boosted voltage to a motor driving circuit in order to avoid insufficiency of a steering assist force has been proposed (for example, see Patent Document 1).
In such a device, when boosting is normally performed by a booster circuit in order to increase a voltage of a battery such that a steering assist force is not insufficient, the booster circuit is increased in size. Further, switching loss is normally generated on the basis of a switching operation of a transistor that constitutes the booster circuit. As a result, energy loss in the booster circuit becomes relatively large. Patent Document 1 discloses an electric power steering device configured to boost a voltage of a battery using a booster circuit only when a duty ratio of a pulse width modulation (PWM) signal for turning on/off a switching element of a motor driving circuit exceeds a predetermined threshold (100%).